science

Photoionization

Photoionization is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photoionization rather than just read about it. In short: Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization.

Key takeaways

Photoionization belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Photoionization to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Photoionization from memory before moving on to harder problems.

Reference excerpt

Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule.

Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The probability of photoionization is related to the photoionization cross section of the species – the probability of an ionization event conceptualized as a hypothetical cross-sectional area. This cross section depends on the energy of the photon (proportional to its wavenumber) and the species being considered i.e. it depends on the structure of the molecular species. In the case of molecules, the photoionization cross-section can be estimated by examination of Franck-Condon factors between a ground-state molecule and the target ion. This can be initialized by computing the vibrations of a molecule and associated cation (post ionization) using quantum chemical software e.g. QChem. For photon energies below the ionization threshold, the photoionization cross-section is near zero. But with the development of pulsed lasers it has become possible to create extremely intense, coherent light where multi-photon ionization may occur via sequences of excitations and relaxations. At even higher intensities (around 1015 – 1016 W/cm2 of infrared or visible light), non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed.

Multi-photon ionization Several photons of energy below the ionization threshold may actually combine their energies to ionize an atom. This probability decreases rapidly with the number of photons required, but the development of very intense, pulsed lasers still makes it possible. In the perturbative regime (below about 1014 W/cm2 at optical frequencies), the probability of absorbing N photons depends on the laser-light intensity I as IN . For higher intensities, this dependence becomes invalid due to the then occurring AC Stark effect. Resonance-enhanced multiphoton ionization (REMPI) is a technique applied to the spectroscopy of atoms and small molecules in which a tunable laser can be used to access an excited intermediate state. Above-threshold ionization (ATI) is an extension of multi-photon ionization where even more photons are absorbed than actually would be necessary to ionize the atom. The excess energy gives the released electron higher kinetic energy than the usual case of just-above threshold ionization. More precisely, the system will have multiple peaks in its photoelectron spectrum which are separated by the photon energies, indicating that the emitted electron has more kinetic energy than in the normal (lowest possible number of photons) ionization case. The electrons released from the target will have approximately an integer number of photon-energies more kinetic energy.

Tunnel ionization When either the laser intensity is further increased or a longer wavelength is applied as compared with the regime in which multi-photon ionization takes place, a quasi-stationary approach can be used and results in the distortion of the atomic potential in such a way that only a relatively low and narrow barrier between a bound state and the continuum states remains. Then, the electron can tunnel through or for larger distortions even overcome this barrier. These phenomena are called tunnel ionization and over-the-barrier ionization, respectively.

Illustrations

Worked examples

Example 1 — a first encounter with Photoionization

Start with the simplest possible case. Write down what Photoionization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photoionization before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photoionization ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photoionization

In research: Photoionization appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photoionization in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photoionization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ionization, Spectroscopy, so understanding it makes those chapters shorter.
In everyday life: Look for Photoionization outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Photoionization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photoionization means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Photoionization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photoionization in simple terms?

Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization.

Why does Photoionization matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photoionization?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photoionization.